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Mesoscale Convective Complexes and Persistent Elongated Convective Systems Over the United States During 1992 and 1993

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Mesoscale Convective Complexes and Persistent Elongated Convective Systems Over the United States During 1992 and 1993 578 MONTHLY WEATHER REVIEW VOLUME 126 Mesoscale Convective Complexes and Persistent Elongated Convective Systems over the United States during 1992 and 1993 CHRISTOPHER J. ANDERSON AND RAYMOND W. A RRITT Department of Agronomy, Iowa State University, Ames, Iowa (Manuscript received 5 February 1997, in ®nal form 12 June 1997) ABSTRACT Large, long-lived convective systems over the United States in 1992 and 1993 have been classi®ed according to physical characteristics observed in satellite imagery as quasi-circular [mesoscale convective complex (MCC)] or elongated [persistent elongated convective system (PECS)] and cataloged. The catalog includes the time of initiation, maximum extent, termination, duration, area of the 2528C cloud shield at the time of maximum extent, signi®cant weather associated with each occurrence, and tracks of the 2528C cloud-shield centroid. Both MCC and PECS favored nocturnal development and on average lasted about 12 h. In both 1992 and 1993, PECS produced 2528C cloud-shield areas of greater extent and occurred more frequently compared with MCCs. The mean position of initiation for PECS in 1992 and 1993 followed a seasonal shift similar to the climatological seasonal shift for MCC occurrences but was displaced eastward of the mean position of MCC initiation in 1992 and 1993. The spatial distribution of MCC and PECS occurrences contain a period of persistent development near 408N in July 1992 and July 1993 that contributed to the extreme wetness experienced in the Midwest during these two months. Both MCC and PECS initiated in environments characterized by deep, synoptic-scale ascent associated with continental-scale baroclinic waves. PECS occurrences initiated more often as vigorous waves exited the inter- mountain region, whereas MCCs initiated more often within a high-amplitude wave with a trough positioned over the northwestern United States and a ridge positioned over the Great Plains. The low-level jet transported moisture into the region of initiation for both MCC and PECS occurrences. The areal extent of convective initiation was limited by the orientation of low-level features for MCC occurrences. 1. Introduction vided a climatological reference for MCC occurrences. These summaries indicate that on average, about 35 Mesoscale convective complexes (MCC; Maddox MCC occurrences have affected the United States an- 1980) are large, long-lived convective systems that ex- nually, with a maximum of 59 in 1985 (Augustine and hibit a quasi-circular cloud shield (infrared temperature Howard 1988) and a minimum of 23 in 1981 (Maddox #2528C) in infrared (IR) satellite imagery. Table 1 lists the speci®c criteria that must be met in order to classify et al. 1982). Typically, the southern Great Plains ex- a convective system as an MCC. These criteria differ periences most of the MCC activity during the spring, slightly from the origional criteria established by Mad- while the MCC activity shifts northward during the sum- dox (1980) in order to simplify the documentation pro- mer. During the fall, the bulk of the MCC activity returns cedure (Augustine and Howard 1988). Research has to the southern Great Plains. The distribution of MCC shown that MCCs can induce adjustments by the large- occurrences during a dry hydrological anomaly in the scale atmospheric circulation (e.g., Fritsch and Maddox central United States in 1983 did not follow the cli- 1981; Menard and Fritsch 1989) and can produce haz- matological trends. Rather, a persistent anticyclone over ardous weather over a large region. Thus, MCCs rep- the central United States hindered the moisture ¯ux into resent a challenging and important forecast problem. the Great Plains that aids the development of MCCs. Annual summaries of MCC occurrences over the This resulted in an increased frequency of smaller quasi- United States (Maddox 1980; Maddox et al. 1982; circular convective system occurrences north of the Rodgers et al. 1983; Rodgers et al. 1985; Augustine and Great Plains throughout the summer and fall (Fritsch et Howard 1988; Augustine and Howard 1991) have pro- al. 1986). These summaries have also contributed knowledge of large-scale features that support MCC development. Au- gustine and Howard (1988) used monthly mean Q-vec- Corresponding author address: Christopher J. Anderson, Depart- tor divergence (Hoskins et al. 1978) to compare the ment of Agronomy, 3010 Agronomy Building, Iowa State University, Ames, IA 50011. synoptic-scale vertical motion for an inactive warm sea- E-mail: [email protected] son (1984) with a very active warm season (1985). Their q 1998 American Meteorological Society Unauthenticated | Downloaded 09/30/21 12:43 PM UTC MARCH 1998 ANDERSON AND ARRITT 579 TABLE 1. Modi®ed MCC de®nition. ful®lls the size and duration criteria, but not the shape Size Continuous cold-cloud shield (IR temperature criterion, of the MCC de®nition. That is, a PECS must #2528C) must have an area $50 000 km2 meet every criterion in Table 1 except the ratio of the Initiation Size de®nition is ®rst satis®ed minor to major axis does not exceed 0.7. While the Duration Size de®nition must be met for a period of internal circulation of convective systems is not ad- $6h Maximum extent Continuous cloud shield (IR temperature dressed with a satellite-based de®nition (as pointed out #2528C) reaches maximum size by Maddox 1980), observational studies (e.g., Fritsch Shape Minor axis/major axis $0.7 at time of and Maddox 1981), as well as the duration and size of maximum extent these convective systems, suggest that most contain a Termination Size de®nition is no longer satis®ed distinct, mesoscale circulation. In order to diminish the likelihood that extremely long lines of convective ele- ments (convective systems not likely to contain a dis- results showed stronger synoptic-scale ascent at 600 and tinct, mesoscale circulation) meet our PECS criteria, we 400 mb for June 1985 (the month with the largest dif- established a lower limit of 0.2 for the ratio of the minor ference in MCC occurrences between 1984 and 1985) to major axis. Our comparative study consists of a tab- suggesting that this period of persistent MCC devel- ulation of PECS and MCC occurrences in 1992 and opment was supported by a persistent, upper-level 1993 and composites of environmental features near the mass±momentum imbalance in the synoptic-scale cir- time of initiation. The tabulation of MCC occurrences culation. Augustine and Howard (1991) addressed low- for 1993 has added to the climatological reference base level features supporting MCC development by com- an account of MCC occurrences during an extreme wet positing 850-mb wind, mixing ratio, geopotential hydrological anomaly in the Great Plains [for an over- height, and 500-mb geopotential height for weeks of view of the 1993 ¯ood see e.g., Kunkel et al. (1994)]. frequent MCC occurrences and weeks of infrequent MCC occurrences in 1986 and 1987. The active weeks were dominated by a deep tropospheric ridge centered 2. Data and methodology over the southeastern United States with low-level mois- a. Data sources ture ¯ux into the central plains region. Most MCC oc- currences were observed near the intersection of a low- Cloud-top characteristics in GOES-7 IR digital sat- level jet (LLJ) and a zonally oriented low-level front. ellite imagery for 1992 and 1993 were analyzed with a The inactive periods were dominated by a 500-mb version of the automated routine developed by Augus- trough over the southeastern United States and low-level tine (1985). The routine searches all pixels within the moisture transport into that region. image that represent cloud-top temperature less than or Unfortunately, large, long-lived non-MCC convective equal to 2528C and uses a harmonic analysis technique system occurrences have not been documented as thor- to compute the best-®t ellipse for the 2528C cloud- oughly despite a potential for hazardous weather similar shield perimeter. The 2528C cloud-shield centroid and to that of MCC occurrences. Only one of the annual eccentricity are computed from the best-®t ellipse. We MCC summaries (Maddox et al. 1982) has included a also view imagery for each system to check that the list of ``non-MCC signi®cant mesoscale convective automated procedure does not detect spurious systems. events.'' The list does not allow for a comparison with We then tabulate the duration of the MCC or PECS the MCC events because satellite characteristics and a occurrence, size of the 2528C cloud shield at maximum description of the life cycle of each event were not extent, and hourly position of the 2528C cloud-shield included. Bartels et al. (1984) performed a satellite- centroid. In addition, we have summarized signi®cant based climatology for 1978±83 over the Stormscale Op- weather associated with each MCC and PECS occur- erational and Research Meteorology (STORM)-Central rence from the National Oceanic and Atmospheric Ad- project domain that included convective systems ex- ministration/National Environmental Satellite, Data, ceeding 250 km along their major axis and 3 h during and Information System (NOAA/NESDIS) publication their life cycle. MCC occurrences and large, long-lived Storm Data. linear convective system occurrences ®t within these Archived data from the National Meteorological Cen- criteria. In their tabulation, large, long-lived linear con- ter (NMC) Global Data Analysis System (GDAS; Kal- vective systems (their ``large cloud lines'') totaled about nay et al. 1990) were used to construct composite anal- one-third of the number of MCC occurrences. yses near the times of MCC and PECS initiation. In We address the lack of information concerning large, both 1992 and 1993, the GDAS used the T80 resolution long-lived non-MCC mesoscale convective system oc- (equivalant to about 200-km grid spacing) Medium currences using a comparative study with MCC occur- Range Forecast Model to provide the background or rences in 1992 and 1993.
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